We have : 3aa1 is divisible by 9

<=> (3 + a + a + 1)  is divisible by 9

<=> (2a + 4)  is divisible by 9

<=>2(a + 2) is divisible by 9

<=> a + 2 is divisible by 9

<=> a = 7

Answer 3771

Peter has the stamp number is:

          40 + 7 = 47 (stamps)

             Answer : 47 stamps

When starting, the motorist goes in :

           60 : 20 = 3 ( hours)

When to return, the motorist goes in :

          60 : 30 = 2 ( hours )

In the entire trip, the motorist goes in :

           3 + 2 = 5 ( hours )

Medium speed of the motorist for the entire trip :

           60 : 5 = 12 (  miles per hour )

                          Answer : 12 miles per hour